Hot corrosion resistant superalloys

ABSTRACT

Under certain circumstances of service, nickel-rich and cobaltrich superalloys are subject to &#39;&#39;&#39;&#39;hot-corrosion&#39;&#39;&#39;&#39; attack. This occurs, for example, when bodies formed from these alloys are operated at elevated temperatures in oxidizing atmospheres in the presence of small amounts of sulfur compounds and salt. It has been found that the presence of a small but effective amount of rare earth oxide in the form of a dispersion of small particles in the alloy effectively reduces this corrosion.

United States atent inventors Alan U. Seybolt Ballston Spa; James L. Walker. Schenectady, both of N.Y.

Appl. No. 888,983

Filed Dec. 29, 1969 Patented Nov. 23, 197 l Assignee General Electric Company HOT CORROSION RESISTANT SUPERALLOYS 10 Claims, No Drawings US. Cl 75/ l 70,

29/1825, 75/l 7| Int. Cl CZZc 19/00 Field of Search 75/ l 7 l I 56] References Cited UNITED STATES PATENTS 2,712,498 7/l955 Gresham et al 75/1 71 ABSTRACT: Under certain circumstances of service, nickelrich and cobalt-rich superalloys are subject to hotcorrosion" attack. This occurs, for example. when bodies formed from these alloys are operated at elevated temperatures in oxidizing atmospheres in the presence of small amounts of sulfur compounds and salt. It has been found that the presence ofa small but effective amount of rare earth oxide in the form of a dispersion of small particles in the alloy efi'cctively reduces this corrosion.

1 nor CORROSION RESISTANT SUPERALLOYS The invention herein described was made in the course ofor under a contract wit the Office of Naval Research. Department of the Navy.

This invention relates to articles made from superalloys which have improved resistance to hot corrosion.

The term superalloys" has been applied to a class of alloys developed for high temperature service where relatively high stresses are encountered. These alloys usually contain as major alloying elements chromium, nickel, cobalt, molybdenum, and tungsten, and as minor constituents, titanium, niobium aluminum, tantalum and others, as well as small amounts of carbon, manganese, silicon and others. Articles are fabricated from such alloys by powder metallurgy techniques, precision casting, or by mechanical forming methods applied to cast ingots. Depending upon the composition of the particular alloy, as well as other considerations, high temperature operations such as melting, casting or sintering may require the use of a vacuum or protective atmosphere environment.

Articles made from such alloys are frequently employed, for example, as gas turbine buckets and the like here they are exposed to high temperature gases and relatively high stresses in operation. The term hot corrosion" has been applied to an accelerated attack upon such articles and particularly those formed from nickel-rich and cobalt-rich alloys, when exposed to hot oxidizing gases containing small amounts of sulfur compounds (particularly Na SO,,) and salt. This condition is particularly troublesome in gas turbine engines operated in marine environments with petroleum-base fuels which contain sulfur.

It would be desirable to improve the resistance of articles made from such alloys to hot corrosion and such is a principal object of this invention.

Briefly stated, and in accordance with one aspect of the invention, it has been found that the addition of elements of the lanthanide series rare earths such as cerium, lanthanum and gadolinium, for example, as alloying additions scavenge the sulfur compounds in the combustion products of the fuel and improve the resistance of nickel-rich superalloy articles to hot corrosion by the formation of one or more stable rare-earthsulfur complexes. Unfortunately, when metallic or elemental rare earth additions are made to such superalloys, an intermetallic phase is formed, such as, for example, Ni Ce in the case of cerium additions. This compound forms a Ni--Ni ,Ce eutectic which in the binary Ni-Ce system melts at l2l0 C. in the presence of other alloying metals in these superalloys, the eutectic temperature drops to even lower values and, upon solidification, these low melting eutectics are segregated in the grain boundaries. This segregation is highly undesirable in that the high temperature properties of the superalloys are degraded. Surprisingly, it has been found that the rare earth additions for the prevention of hot corrosion should be present in the superalloys as their oxides. It is only in this form that the rare earth elements appear to be effective as scavengers for the sulfur compounds. The scavenging occurs by the formation of a rare earth oxysulfide such as, for example, Ce- O S. When oxides are used, the formation and segregation of the intermetallic compound eutectics are avoided and the mechanical properties are not thereby degraded While powder metallurgical techniques may be and have been used successfully to fabricate articles from superalloy compositions modified by such rare earth oxide additions, it is apparent that melting and casting techniques may be used which permit such oxide modified superalloy articles to be made. It will be appreciated that as between articles made by powder metallurgical techniques from a given superalloy and similar articles made by melting and casting techniques fromthe same composition, the cast articles usually have the more desirable high temperature properties.

A number of specific tests were made on such superalloys and the following results are given as illustrative of the beneficial effects of the rare earth oxide additions. As indicated in the table, the test specimens were formed from castings and conventionally manufactured sintered powder metallurgy bodies, all formed from an alloy having the nominal composition of about 7.5 weight percent cobalt, 6.1 percent chromium, 5.4 percent aluminum, 1.0 percent titanium, 2.0 percent molybdenum, 5.8 percent tungsten, 0.5 percent niobium, 9.0 percent tantalum, 0.13 percent carbon and the balance nickel with the usual small amounts of impurities customarily found in such materials. These specimens were subjected to a temperature of 1725 F. for 50 hours in a burner flame using .IP-S aircraft jet engine fuel into which parts per million sodium chloride was ingested. The test specimens were cylindrical in form and after the exposure to the flame, they were sectioned and the extent of the attack by hot corrosion was measured and expressed in tenns of metal loss and sulfide penetration in mils per diameter.

Sulfide Metal Loss Penetration Sample Dopant (mils/dia.) (mils/dia.)

Cast 0 34.2 41.7 sintered 0 36.0 43.4 Sintered 0.5Ce0, 0.6 L9 Sintered 0.5l..a,0 0.6 2.1

The dramatic improvement in the hot corrosion properties of the nickel-base superalloy by the relatively minor addition of the rare earth oxide thereto is demonstrated by these test results. It will be apparent to those skilled in the art that a very finely divided, evenly distributed dispersion of the rare earth oxides is preferable. These oxide additions may be made conveniently as finely divided simple oxide powders or as oxide salts which decompose to the simple oxides when the body is formed and sintered. As previously stated, because cast structures or structures formed by casting and mechanically working are usually preferred because of higher mechanical properties, and because such a distribution of finely divided rare earth oxide particles may not be achieved by simply mixing such particles in a molten alloy bath, a melting and treatment schedule such as the following may be employed to produce a superalloy casting or ingot having the desired oxide distribution.

Where the alloy to be cast is to contain substantial amounts of titanium and aluminum, both of which form very stable oxides which are not readily reducible, those components of the alloy such as nickel, cobalt, chromium, etc. which do form readily reducible oxides are melted with the appropriate amount of elemental rare earth or rare earth alloy such as, for example, Misch metal. The melt is then subjected to an oxidizing treatment by exposing it to gaseous oxygen or by the addition of MO, or the like to an extent sufficient to substantially completely oxidize the rare earth addition. The rare earths will preferentially form oxides in the presence of the other components of the melt and this may be facilitated by vigorous stirring of the melt by conventional methods such as induction stirring, for example, during the partial oxidation step. The partially oxidized melt is then subjected to deoxidation by treatment with hydrogen while stirring whereupon all the oxides in the melt are reduced except the rare earth oxides. The final additions of titanium and aluminum are then made and the melt is poured into a mold using practices which are conventionally employed with such alloys.

As an alternative, a powder metallurgy compact comprised of nickel powder or nickel alloy powder with an appropriate amount of rare earth oxide may be employed. In this case, an appropriate melt composition is prepared and a powder metal compact material composed of, for example, 2 to 5 micron nickel or nickel alloy powder containing about 1 to 5 percent rare earth oxide is employed, preferably as a heat end addition and utilizing vigorous induction stirring to assure adequate mixing before pouring.

in the foregoing description, reference has been made to nickel-base or nickel-rich superalloys. it will be recognized by those skilled in the art that these terms are used in the art to define superalloys which contain more than 50 percent by weight nickel. It has been found that n analogous group of superalloys based on cobalt and referred to in the art as cobaltbase" or cobalt-rich superalloys are susceptible to the same type of hot-corrosion attack as the nickel-base alloys, and that these alloys contain more than 50 percent by weight cobalt. The same rare earth oxide addiitons may be utilized to prevent or mitigate hot corrosion in these cobalt-base superalloys. The efiective lower limit of these rare earth oxide additions is of the order of 0.1 to 0.2 weight percent and if more than about 2.0 weight percent is used, the ductility of the material becomes affected.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An article of manufacture comprised of a superalloy which is resistant to attack by hot corrosion consisting of a matrix composed of a "superalloy selected from the group consisting of the nickel-base superalloys and the cobalt base superalloys, said matrix containing an evenly distributed dispersion of sulfur scavenging particles comprising finely divided rare earth oxides.

2. An article of manufacture as set forth in claim I wherein said rare earth oxide particles are present in said matrix in a small but effective amount up to about 2.0 weight percent.

3. An article of manufacture as set forth in claim I wherein said rare earth oxide particles are present in said matrix in amounts of about 0.2 to 2.0 weight percent.

4. An article of manufacture as set forth in claim 1 wherein said matrix is composed of a nickel base superalloy and said rare earth oxide particles are present in an amount up to 2.0 percent by weight.

5. An article of manufacture as set forth in claim 4 wherein said rare earth oxide particles are present in an amount of at least 0.2 percent by weight.

6. An article of manufacture as set forth in claim 1 wherein said matrix is composed of a cobalt base superalloy and said rare earth oxide particles are present in an amount between about 0.2 and 2.0 percent by weight.

7. An article of manufacture as set forth in claim 4 wherein said rare earth oxides is cerium oxide.

8. An article of manufacture as set forth in claim 4 wherein said rare earth oxide is lanthanum oxide.

9. An article of manufacture as set forth in claim 4 wherein said rare earth oxide is gadolinium oxide.

10. An article of manufacture as set forth in claim 4 wherein said rare earth oxide is composed of the oxide of Misch metal. 

2. An article of manufacture as set forth in claim 1 wherein said rare earth oxide particles are present in said matrix in a small but effective amount up to about 2.0 weight percent.
 3. An article of manufacture as set forth in claim 1 wherein said rare earth oxide particles are present in said matrix in amounts of about 0.2 to 2.0 weight percent.
 4. An article of manufacture as set forth in claim 1 wherein said matrix is composed of a nickel base superalloy and said rare earth oxide particles are present in an amount up to 2.0 percent by weight.
 5. An article of manufacture as set forth in claim 4 wherein said rare earth oxide particles are present in an amount of at least 0.2 percent by weight.
 6. An article of manufacture as set forth in claim 1 wherein said matrix is composed of a cobalt base superalloy and said rare earth oxide particles are present in an amount betweeN about 0.2 and 2.0 percent by weight.
 7. An article of manufacture as set forth in claim 4 wherein said rare earth oxides is cerium oxide.
 8. An article of manufacture as set forth in claim 4 wherein said rare earth oxide is lanthanum oxide.
 9. An article of manufacture as set forth in claim 4 wherein said rare earth oxide is gadolinium oxide.
 10. An article of manufacture as set forth in claim 4 wherein said rare earth oxide is composed of the oxide of Misch metal. 